Stem cell research have demonstrated that intravenously administered stem cells isolated from adult bone marrow and muscle can be recruited into regenerating muscles which raises the hope for systemic cellbased therapy for DMD patients. However, the stem cell-mediated muscle regeneration appears to be inefficient. A recent study has shown that the stem cell-mediated muscle regeneration undergoes a biological progression after bone marrow transplantation. Bone marrow-derived cells (BMDC) first become quiescent satellite cells residing in the muscle and then they proliferate to form myofibers in response to regenerating cues (Blau's paper). Thus the low efficiency of muscle regeneration from the transplanted stem cells which has been seen could be caused by 1): a failure to compete with endogenous satellite cells for proliferation, 2): lack of a strong regenerating stimulus, or 3): limited proliferating capacity. In the proposed study, we will test the hypothesis that efficient muscle regeneration from intravenously administered stem cells can be achieved by a combination of completely blocking endogenous satellite cell growth, vigorously stimulating regeneration and improving the proliferation capacity of the stem cell-derived satellite cells. Specific aim 1.-1.1. To determine the optimum irradiation dose needed to eliminate endogenous satellite cells in mdx mice. 1.2. To evaluate the ability of intravenously administered M SP cells to repopulate the satellite cell niche compared to bone marrow derived cells. 1.3. To test whether the M SP cell-derived satellite cells can efficiently correct murine muscular dystrophy Specific aim 2. To determine if dystrophic M SP cells that have been genetically corrected with an HSV/AAV hybrid vector are as efficient as wt M SP cells in correcting the mdx phenotype. Specific aim 3. To determine if it is possible to improve the proliferation capacity of donor-derived satellite cells by transducing M SP cells with: 3.1 a cDNA expressing the muscle isoform of insulin-like growth factor I (mlGF-I) or 3.2 a cDNA expressing HoxB4.